Pre-precipitate clusters and precipitation processes in Al–Mg–Si alloys

In the continuing drive for automobile weight reduction, the 6000 series Al-Mg-Si alloys are considered the most promising candidates for age-hardenable bodysheet materials. The solute clusters and the metastable precipitates in aged Al-Mg-Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg-Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of {beta}{double_prime} after 10 h artificial aging at 175 C varies depending on pre-aging conditions, i.e. pre-aging at 70 C increases the number density of the {beta}{double_prime} precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70 C serve as nucleation sites for the {beta}{double_prime} in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the {beta}{double_prime} precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al-Mg{sub 2}Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al-Mg-Si alloys is discussed.